A conventional combustible gas turbine engine includes a compressor, a combustor, and a turbine. The compressor compresses ambient air. The combustor combines the compressed air with a fuel and ignites the mixture creating combustion products defining a working gas. The working gases travel to the turbine. Within the turbine are a series of rows of stationary vanes and rotating blades. Each pair of rows of vanes and blades is called a stage. Typically, there are multiple stages in a turbine. The rotating blades are coupled to a shaft and disc assembly. As the working gases expand through the turbine, the working gases cause the blades, and therefore the shaft and disc assembly, to rotate.
A diffuser may be positioned downstream from the turbine. The diffuser comprises a duct whose cross-sectional area increases with distance. Due to its increasing cross sectional area, the diffuser functions to decelerate the exhaust gases. Hence, the kinetic energy of the exhaust gases decreases while the pressure of the exhaust gases increases. The greater the pressure recovery before the exhaust gases exit the diffuser, the lower the exhaust gas pressure is at the last turbine stage. The lower the pressure at the last turbine stage, the greater the pressure ratio across the turbine and the greater the work from the turbine.
It is desirable to produce a large increase of pressure and decrease of exhaust gas flow velocity from the inlet to the exit of the diffuser. Diffusion within a diffuser can be reduced where gas flow separates from the diffuser walls. Hence, it is desirable to minimize gas flow separation from the walls of a diffuser.